Simulation and Modeling of Telocytes Behavior in Signaling and Intercellular Communication Processes

The Potential of Berberine to Target Telocytes in Rabbit Heart

A brand-new class of interstitial cells, called telocytes, has been detected in the heart. Telocytes can connect and transmit signals to almost all cardiomyocytes; this is highly interrelated with the occurrence and development of heart diseases. Modern studies have shown that berberine has a therapeutic effect on cardiovascular health. However, berberine's mechanism of action on the cardiovascular system through cardiac telocytes is unclear. Interestingly, 5 µm of berberine remarkably decreased the concentration of intracellular calcium and membrane depolarization in cultured telocytes, upregulated the expression of CX43 and β-catenin, and downregulated the expressions of TRPV4 and TRPV1. Here, telocytes were identified in the vascular adventitia and intima, endocardium, myocardium, adventitia, and heart valves. Moreover, telocytes were broadly dispersed around cardiac vessels and interacted directly through gap junctions and indirectly through extracellular vesicles. Together, cardiac telocytes interact with berberine and then deliver drug information to the heart. Telocytes may be an essential cellular target for drug therapy of the cardiovascular system.

Stromal cell-derived factor 1 (SDF-1) increases the number of telocytes in ex vivo and in vitro assays

Telocytes are interstitial cells that are present in various tissues, have long cytoplasmic projections known as telopodes, and are classified as CD34+ cells. Telopodes form extensive networks that permeate the stroma, and there is evidence that these networks connect several stromal cell types, giving them an important role in intercellular communication and the maintenance of tissue organisation. Data have also shown that these networks can be impaired and the number of telocytes reduced in association with many pathological conditions such as cancer and fibrosis. Thus, techniques that promote telocyte proliferation have become an important therapeutic target. In this study, ex vivo and in vitro assays were conducted to evaluate the impact on prostatic telocytes of SDF-1, a factor involved in the proliferation and migration of CD34+ cells. SDF-1 caused an increase in the number of telocytes in explants, as well as morphological changes that were possibly related to the proliferation of these cells. These changes involved the fusion of telopode segments, linked to an increase in cell body volume. In vitro assays also showed that SDF-1 enriched prostate stromal cells with telocytes. Altogether, the data indicate that SDF-1 may offer promising uses in therapies that aim to increase the number of telocytes. However, further studies are needed to confirm the efficiency of this factor in different tissues/pathological conditions.

Cellular Evidence for Telocytes Mediating Electroacupuncture to Ameliorate Obesity in Mice

Electroacupuncture has been generally applied to target obesity, the principle of which is based on the meridian in traditional Chinese medicine. Although Telocytes (TCs) have been reported as the potential essence of meridians, their specific role in the electroacupuncture treatment of obesity remains unclear. Thus, we investigated the cellular evidence for TC-mediated electroacupuncture to alleviate obesity. Mice were divided into three groups as follows: electroacupuncture group (EA), control group (CG), and normal group (NG). The present study showed that the weight of perirenal white adipose tissue (rWAT), the serum level of total cholesterol, and the low-density lipoprotein cholesterol were all significantly decreased after electroacupuncture. Ultrastructurally, the prolongations (telopodes, Tps) of TCs were in direct contact with adipocytes, and lipid droplets were distributed on the surface of Tps. The proportions of double-positive fluorescent areas of TCs (CD34 and PDGFRα) were significantly elevated with concomitant elongated Tps in EA mice, as compared to those in CG mice. The expression of Cx43 and CD63 (gap junction and exosome markers) was significantly enhanced. These characteristics facilitated the transmission of electroacupuncture stimulation from skin to rWAT. We conclude that electroacupuncture relieved obesity by activating TCs morphologically, upregulating the gap junctions between TCs, and increasing the exosomes around TCs.

Telocytes/CD34+ Stromal Cells in the Normal, Hyperplastic, and Adenomatous Human Parathyroid Glands

Telocytes/CD34+ stromal cells (TCs/CD34+ SCs) have been studied in numerous organs and tissues, but their presence and characteristics in the parathyroid glands have not been explored. Using immunological and ultrastructural procedures, we assess the location, arrangement, and behavior of TCs/CD34+ SCs in normal human parathyroids, during their development and in their most frequent pathologic conditions. In normal parathyroids, TCs/CD34+ SCs show a small somatic body and long thin processes with a moniliform aspect, form labyrinthine systems, connect other neighboring TCs/CD34+ SCs, vessels, adipocytes, and parenchymal cells directly or by extracellular vesicles, and associate with collagen I. TCs/CD34+ SCs and collagen I are absent around vessels and adipocytes within parenchymal clusters. In developing parathyroids, TCs/CD34+ SC surround small parenchymal nests and adipocytes. In hyperplastic parathyroids, TCs/CD34+ SCs are prominent in some thickened internodular septa and surround small extraglandular parenchymal cell nests. TCs/CD34+ SCs are present in delimiting regions with compressed parathyroids and their capsule in adenomas but absent in most adenomatous tissue. In conclusion, TCs/CD34+ SCs are an important cellular component in the human parathyroid stroma, except around vessels within parenchymal nests. They show typical characteristics, including those of connecting cells, are present in developing parathyroids, and participate in the most frequent parathyroid pathology, including hyperplastic and adenomatous parathyroids.

Morphological and histochemical identification of telocytes in adult yak epididymis

Telocytes (TCs) are a newly discovered type of mesenchymal cell that are closely related to the tissue's internal environment. The study aimed to investigate the morphological identification of TCs in the epididymis of adult yak and their role in the local microenvironment. In this study, transmission electron microscopy (TEM), scanning electron microscopy, immunofluorescence, qRT-PCR, and western blotting were used to analyze the cell morphology of TCs. The results showed that there are two types of TCs in the epididymal stroma of yak by TEM; one type is distributed around the capillaries with full cell bodies, longer TPs, and a large number of secretory vesicles; the other is distributed outside the basement membrane with irregularly long, striped, large nuclei and short telopodes (TPs). In addition, these TCs formed complex TC cell networks through TPs with epididymal interstitial capillaries and basal fibroblasts. TCs often appear near the capillaries and basement membrane by special staining. The surface markers of TCs (CD34, vimentin, and CD117) were positively expressed in the epididymal stroma and epithelium by immunohistochemistry, and immunofluorescence co-expression of vimentin + CD34 and CD117 + CD34 was observed on the surface of TCs. The trends in the mRNA and protein expression of TCs surface markers revealed expression was highest in the caput epididymis. In summary, this is first report of TCs in the epididymis of yak, and two phenotypes of TCs were observed. The existence and distribution characteristics of TCs in the epididymis of plateau yaks provide important clues for further study of the adaptation to reproductive function in the plateau.

Delimiting CD34+ Stromal Cells/Telocytes Are Resident Mesenchymal Cells That Participate in Neovessel Formation in Skin Kaposi Sarcoma

Kaposi sarcoma (KS) is an angioproliferative lesion in which two main KS cell sources are currently sustained: endothelial cells (ECs) and mesenchymal/stromal cells. Our objective is to establish the tissue location, characteristics and transdifferentiation steps to the KS cells of the latter. For this purpose, we studied specimens of 49 cases of cutaneous KS using immunochemistry and confocal and electron microscopy. The results showed that delimiting CD34+ stromal cells/Telocytes (CD34+SCs/TCs) in the external layer of the pre-existing blood vessels and around skin appendages form small convergent lumens, express markers for ECs of blood and lymphatic vessels, share ultrastructural characteristics with ECs and participate in the origin of two main types of neovessels, the evolution of which gives rise to lymphangiomatous or spindle-cell patterns-the substrate of the main KS histopathological variants. Intraluminal folds and pillars (papillae) are formed in the neovessels, which suggests they increase by vessel splitting (intussusceptive angiogenesis and intussusceptive lymphangiogenesis). In conclusion, delimiting CD34+SCs/TCs are mesenchymal/stromal cells that can transdifferentiate into KS ECs, participating in the formation of two types of neovessels. The subsequent growth of the latter involves intussusceptive mechanisms, originating several KS variants. These findings are of histogenic, clinical and therapeutic interest.

Comparison of the Behavior of Perivascular Cells (Pericytes and CD34+ Stromal Cell/Telocytes) in Sprouting and Intussusceptive Angiogenesis

Perivascular cells in the pericytic microvasculature, pericytes and CD34+ stromal cells/telocytes (CD34+SCs/TCs), have an important role in angiogenesis. We compare the behavior of these cells depending on whether the growth of endothelial cells (ECs) from the pre-existing microvasculature is toward the interstitium with vascular bud and neovessel formation (sprouting angiogenesis) or toward the vascular lumen with intravascular pillar development and vessel division (intussusceptive angiogenesis). Detachment from the vascular wall, mobilization, proliferation, recruitment, and differentiation of pericytes and CD34+SCs/TCs, as well as associated changes in vessel permeability and functionality, and modifications of the extracellular matrix are more intense, longer lasting over time, and with a greater energy cost in sprouting angiogenesis than in intussusceptive angiogenesis, in which some of the aforementioned events do not occur or are compensated for by others (e.g., sparse EC and pericyte proliferation by cell elongation and thinning). The governing mechanisms involve cell-cell contacts (e.g., peg-and-socket junctions between pericytes and ECs), multiple autocrine and paracrine signaling molecules and pathways (e.g., vascular endothelial growth factor, platelet-derived growth factor, angiopoietins, transforming growth factor B, ephrins, semaphorins, and metalloproteinases), and other factors (e.g., hypoxia, vascular patency, and blood flow). Pericytes participate in vessel development, stabilization, maturation and regression in sprouting angiogenesis, and in interstitial tissue structure formation of the pillar core in intussusceptive angiogenesis. In sprouting angiogenesis, proliferating perivascular CD34+SCs/TCs are an important source of stromal cells during repair through granulation tissue formation and of cancer-associated fibroblasts (CAFs) in tumors. Conversely, CD34+SCs/TCs have less participation as precursor cells in intussusceptive angiogenesis. The dysfunction of these mechanisms is involved in several diseases, including neoplasms, with therapeutic implications.

Telocytes and Macrophages in the Gut: From Morphology to Function. Do the Two Cell Types Interact with Each Other? Which Helps Which?

Telocytes and macrophages are ubiquitous cells located in loose connective tissues and share the same mesenchymal origin. Despite these common elements, depending on where they reside, these two cell types are profoundly different in terms of their morphology and functions. The purpose of this review is to provide an update on the knowledge regarding telocytes and macrophages in the gut, where their presence and significance have long been underestimated or misunderstood. The focus will be on the possibility that these two cell types interact with each other and on the potential meaning of these interactions. Based on the complexity of the topic, the variety of possible methodological approaches and the expertise of the author, the point of view in the discussion of the literature data will be mainly morphological. Furthermore, considering the relatively recent period in which these cell types have acquired a primary role in gastrointestinal functions, the attention will be greatly confined to those articles published in the last decade. The microbiota, another main protagonist in this context, will be mentioned only in passing. It is hoped that this review, although not exhaustive, will highlight the importance of macrophages and telocytes in the complex mechanisms that ensure intestinal functions.

Telocytes and Their Structural Relationships With the Sperm Storage Tube and Surrounding Cell Types in the Utero-Vaginal Junction of the Chicken

Telocytes (TCs) are a new type of mesenchymal cells that have been discovered recently in many organs and tissues. However, studies of TCs in the avian reproductive system are still at the beginning. Chickens are one of the world's most popular domesticated animals, providing inexpensive but valuable proteins and nutrients from chickens and eggs to nourish the human bodies. Chickens have important scientific value; thus, understanding the reproductive system regulations seems to be important. The utero-vaginal junction is involved in the regulation of sperm storage. The sperm storage tube (SST) in the utero-vaginal junction stores sperm. The purpose of this study was to investigate the existence of TCs in the utero-vaginal junction of the chicken, and their structural relationships with the sperm storage tube and surrounding cell types. We studied the morphology, ultrastructure, and immune characterization of TCs.

Telocytes inside of the peripheral nervous system - a 3D endoneurial network and putative role in cell communication

In this paper, we developed the hypothesis concerning the reasons to assimilate endoneurial fibroblast-like dendritic phenotype [shortly termed endoneurial dendritic cells (EDCs)] to the endoneurial telocytes (TCs). We reviewed the literature concerning EDCs status and report our observations on ultrastructure and some immune electron microscopic aspects of the cutaneous peripheral nerves. Our data demonstrate that EDCs long time considered as fibroblasts or fibroblast-like, with an ovoidal nucleus and one or more moniliform cell extensions [telopodes (Tps)], which perform homocellular junctions, also able to shed extracellular microvesicles can be assimilated to TC phenotype. Sometimes, small profiles of basement membrane accompany to some extent Tps. Altogether data resulted from scientific literature and our results strength the conclusion EDCs are really TCs inside of the peripheral nervous system. The inner three-dimensional (3D) network of endoneurial TCs by their homo- and heterocellular communications appears as a genuine cell-to-cell communication system inside of each peripheral nerve.

Morphological Evidence of Telocytes in Skeletal Muscle Interstitium of Exercised and Sedentary Rodents

Skeletal muscle atrophy, resulting from states of hypokinesis or immobilization, leads to morphological, metabolic, and functional changes within the muscle tissue, a large variety of which are supported by the stromal cells populating the interstitium. Telocytes represent a recently discovered population of stromal cells, which has been increasingly identified in several human organs and appears to participate in sustaining cross-talk, promoting regenerative mechanisms and supporting differentiation of local stem cell niche. The aim of this morphologic study was to investigate the presence of Telocytes in the tibialis anterior muscle of healthy rats undergoing an endurance training protocol for either 4 weeks or 16 weeks compared to sedentary rats. Histomorphometric analysis of muscle fibers diameter revealed muscle atrophy in sedentary rats. Telocytes were identified by double-positive immunofluorescence staining for CD34/CD117 and CD34/vimentin. The results showed that Telocytes were significantly reduced in sedentary rats at 16 weeks, while rats subjected to regular exercise maintained a stable Telocytes population after 16 weeks. Understanding of the relationship between Telocytes and exercise offers new chances in the field of regenerative medicine, suggesting possible triggers for Telocytes in sarcopenia and other musculoskeletal disorders, promoting adapted physical activity and rehabilitation programmes in clinical practice.

Cd34+ Stromal Cells/Telocytes in Normal and Pathological Skin

We studied CD34+ stromal cells/telocytes (CD34+SCs/TCs) in pathologic skin, after briefly examining them in normal conditions. We confirm previous studies by other authors in the normal dermis regarding CD34+SC/TC characteristics and distribution around vessels, nerves and cutaneous annexes, highlighting their practical absence in the papillary dermis and presence in the bulge region of perifollicular groups of very small CD34+ stromal cells. In non-tumoral skin pathology, we studied examples of the principal histologic patterns in which CD34+SCs/TCs have (1) a fundamental pathophysiological role, including (a) fibrosing/sclerosing diseases, such as systemic sclerosis, with loss of CD34+SCs/TCs and presence of stromal cells co-expressing CD34 and αSMA, and (b) metabolic degenerative processes, including basophilic degeneration of collagen, with stromal cells/telocytes in close association with degenerative fibrils, and cutaneous myxoid cysts with spindle-shaped, stellate and bulky vacuolated CD34+ stromal cells, and (2) a secondary reactive role, encompassing dermatitis—e.g., interface (erythema multiforme), acantholytic (pemphigus, Hailey–Hailey disease), lichenoid (lichen planus), subepidermal vesicular (bullous pemphigoid), psoriasiform (psoriasis), granulomatous (granuloma annulare)—vasculitis (leukocytoclastic and lymphocytic vasculitis), folliculitis, perifolliculitis and inflammation of the sweat and sebaceous glands (perifolliculitis and rosacea) and infectious dermatitis (verruca vulgaris). In skin tumor and tumor-like conditions, we studied examples of those in which CD34+ stromal cells are (1) the neoplastic component (dermatofibrosarcoma protuberans, sclerotic fibroma and solitary fibrous tumor), (2) a neoplastic component with varying presentation (fibroepithelial polyp and superficial myxofibrosarcoma) and (3) a reactive component in other tumor/tumor-like cell lines, such as those deriving from vessel periendothelial cells (myopericytoma), epithelial cells (trichoepithelioma, nevus sebaceous of Jadassohn and seborrheic keratosis), Merkel cells (Merkel cell carcinoma), melanocytes (dermal melanocytic nevi) and Schwann cells (neurofibroma and granular cell tumor).

Telocytes and Other Interstitial Cells: From Structure to Function

The Special Issue, "Telocytes and Other Interstitial Cells: From Structure to Function" of the International Journal of Molecular Sciences, is dedicated to recent progress in research on interstitial cells [...].

Telocytes in the atherosclerotic carotid artery: Immunofluorescence and TEM evidence

Telocytes, which possess distinct body shapes and long telopodes, are allocated in the vascular wall. As a fundamental cell type, telocytes construct a three-dimensional network to form a support structure for the artery. This study aims to characterize the morphology and ultrastructure of telocytes in atherosclerotic arteries. ApoE gene-deficient mice were selected as the atherosclerosis animal model and fed a high-fat diet for at least 12 weeks, and immunofluorescence assays and transmission electron microscopy techniques were used to observe changes in telocytes in atherosclerotic arteries. By immunofluorescence staining, CD34, CD117 and PDGFR-α were positive compared with negative CD28/vimentin in telocytes in the atherosclerotic carotid artery, and they were distributed in the tunica intima and tunica adventitia. Under transmission electron microscopy, the bodies of telocytes became larger, while telopodes became shorter compared with their normal condition, and a mass of lipidosomes was present during the progression of atherosclerosis. These results demonstrate that immunofluorescence with TEM is the critical method for identifying TCs and that steatosis of TCs is a reason for atherosclerotic artery dysfunction.

Telocytes/CD34+ Stromal Cells in Pathologically Affected White Adipose Tissue

We studied telocytes/CD34+ stromal cells (TCs/CD34+SCs) in pathologically affected white adipose tissue after briefly examining them in normal fat. To this aim, we reviewed pathological processes, including original contributions, in which TCs/CD34+SCs are conserved, increased, and lost, or acquire a specific arrangement. The pathologic processes in which TCs/CD34+SCs are studied in adipose tissue include inflammation and repair through granulation tissue, iatrogenic insulin-amyloid type amyloidosis, non-adipose tissue components (nerve fascicles and fibres in neuromas and hyperplastic neurogenic processes) and tumours (signet ring carcinoma with Krukenberg tumour and colon carcinoma) growing in adipose tissue, adipose tissue tumours (spindle cell lipoma, dendritic fibromyxolipoma, pleomorphic lipoma, infiltrating angiolipoma of skeletal muscle and elastofibrolipoma), lipomatous hypertrophy of the interatrial septum, nevus lipomatosus cutaneous superficialis of Hoffman–Zurhelle and irradiated adipose tissue of the perirectal and thymic regions. Two highly interesting issues emerged: (1) whether the loss of CD34 expression in TCs/CD34+SCs is by changes in marker expression or the disappearance of these cells (the findings suggest the first possibility) and (2) whether in some invasive and metastatic malignant tumours, TCs/CD34+SCs that completely surround neoplastic cells act as nurse and/or isolating cells. Further studies are required on adipose tissue TCs/CD34+SCs, mainly in lipomatosis and obesity.

Mechanisms of interactions between lung-origin telocytes and mesenchymal stem cells to treat experimental acute lung injury

Acute lung injury is a serious form and major cause of patient death and still needs efficient therapies. The present study evidenced that co-transplantation of mesenchymal stem cells (MSCs) and telocytes (TCs) improved the severity of experimental lung tissue inflammation, edema, and injury, where TCs increased MSCs migration into the lung and the capacity of MSCs proliferation and movement. Of molecular mechanisms, Osteopontin-dominant networks were active in MSCs and TCs, and might play supportive and nutrimental roles in the interaction between MSCs and TCs, especially activated TCs by lipopolysaccharide. The interaction between epidermal growth factor and its receptor from MSCs and TCs could play critical roles in communications between MSCs and TCs, responsible for MSCs proliferation and movement, especially after inflammatory activation. Our studies provide the evidence that TCs possess nutrimental and supportive roles in implanted MSCs, and co-transplantation of MSCs and TCs can be a new alternative in the therapy of acute lung injury.

Presence/Absence and Specific Location of Resident CD34+ Stromal Cells/Telocytes Condition Stromal Cell Development in Repair and Tumors

CD34+ stromal cells/telocytes (CD34+SCs/TCs) can have a role as mesenchymal precursor cells. Our objective is to assess whether the myofibroblastic stromal cell response in repair and in desmoplastic reactions in tumors depend on the presence or absence of resident CD34+SCs/TCs in specific regions/layers of an organ and on the location of their possible subpopulations. For this purpose, using conventional and immunohistochemical procedures, we studied specimens of (a) acute cholecystitis, with early repair phenomena (n: 6), (b) surgically resected segments of colon tattooed with India ink during previous endoscopic removal of malignant polyps, with macrophage infiltration and stromal cell reaction (n: 8) and (c) infiltrative adenocarcinomas of colon, with desmoplastic reaction (n: 8). The results demonstrated (a) stromal myofibroblastic reaction during repair and tumor desmoplasia in most regions in which resident CD34+SCs/TCs are present, (b) absence of stromal myofibroblastic reaction during repair in the mucosa of both organs in which resident CD34+SCs/TCs are absent and (c) permanence of CD34+SCs/TCs as such, without myofibroblastic response, in smooth muscle fascicles, nerves, and Meissner and Auerbach plexuses, in which the CD34+SCs/TCs mainly undergo reactive phenomena. Therefore, the development of activated αSMA+ myofibroblasts in these conditions requires the presence of resident CD34+SCs/TCs and depends on their location. In conclusion, the facts support the hypotheses that CD34+SCs/TCs participate in the origin of myofibroblasts during repair and tumor stroma formation, and that there is a heterogeneous population of resident CD34+SCs/TCs with different roles.

Explant culture: A relevant tool for the study of telocytes

Telocytes are cells present in the stroma of various tissues including the prostate. The detection of telocytes is still very much dependent on obtaining ultrastructural data that show the presence of telopodes, which are cytoplasmic projections that alternate between dilated regions, the podoms, and thin segments, the podomers. These structures are the distinctive characteristics of the telocytes. Thus, in vitro assays are important for the study of telocytes, which are more easily identified in culture, which also enables the experimental manipulation of these cells. The isolation of telocytes per se does not allow the analysis of the behavior of these cells in relation to other cell types in a given organ. In this sense, in the prostate, explants could be a useful tool for the study of telocytes. The present study obtained prostatic explants and evaluated the influence of recombinant proteins, scattering factor (SCF) and stromal-derived factor 1 (SDF-1), which could impact on the migration of CD34-positive cells. Telocytes migrate out of explants and SDF-1 stimulates the proliferation and formation of telocyte networks in vitro. Telocytes are not smooth muscle cell progenitors in the prostate; on the contrary, they are CD90- and CD44-negative cells and, hence, have limited progenitor capacity. The present study demonstrated that explants are useful tools to elucidate the nature of telocytes and their functions.

A Two-Step Immunomagnetic Microbead-Based Method for the Isolation of Human Primary Skin Telocytes/CD34+ Stromal Cells

Telocytes (TCs), commonly referred to as TCs/CD34+ stromal cells, are a peculiar type of interstitial cells with distinctive morphologic traits that are supposed to exert several biological functions, including tissue homeostasis regulation, cell-to-cell signaling, immune surveillance, and reparative/regenerative effects. At present, the majority of studies investigating these cells are mainly descriptive and focus only on their morphology, with a consequent paucity of functional data. To gain relevant insight into the possible functions of TCs, in vitro analyses are clearly required, but currently, the protocols for TC isolation are only at the early stages and not fully standardized. In the present in vitro study, we describe a novel methodology for the purification of human primary skin TCs through a two-step immunomagnetic microbead-based cell separation (i.e., negative selection for CD31 followed by positive selection for CD34) capable of discriminating these cells from other connective tissue-resident cells on the basis of their different immunophenotypic features. Our experiments clearly demonstrated that the proposed method allows a selective purification of cells exhibiting the peculiar TC morphology. Isolated TCs displayed very long cytoplasmic extensions with a moniliform silhouette (telopodes) and presented an immunophenotypic profile (CD31−/CD34+/PDGFRα+/vimentin+) that unequivocally differentiates them from endothelial cells (CD31+/CD34+/PDGFRα−/vimentin+) and fibroblasts (CD31−/CD34−/PDGFRα+/vimentin+). This novel methodology for the isolation of TCs lays the groundwork for further research aimed at elucidating their functional properties and possible translational applications, especially in the field of regenerative medicine.

The Telocytes: Ten Years after Their Introduction in the Scientific Literature. An Update on Their Morphology, Distribution, and Potential Roles in the Gut

Ten years ago, the term 'telocyte' was introduced in the scientific literature to describe a 'new' cell type described in the connective tissue of several organs by Popescu and Faussone-Pellegrini (2010). Since then, 368 papers containing the term 'telocyte' have been published, 261 of them in the last five years. These numbers underscore the growing interest in this cell type in the scientific community and the general acceptance of the name telocyte to indicate this interstitial cell. Most of these studies, while confirming the importance of transmission electron microscopy to identify the telocytes with certainty, highlight the variability of their immune phenotypes. This variability was interpreted as due to (i) the ability of the telocytes to adapt to the different sites in which they reside; (ii) the distinct functions they are likely to perform; and (iii) the existence of telocyte subtypes. In the present paper, an overview of the last 10 years of literature on telocytes located in the gut will be attempted, confining the revision to the morphological findings. A distinct chapter will be dedicated to the recently hypothesized role of the telocytes the intestinal mucosa. Through this review, it will be shown that telocytes, despite their variability, are a unique interstitial cell.

Telocytes in the Normal and Pathological Peripheral Nervous System

We studied telocytes/CD34+ stromal cells in the normal and pathological peripheral nervous system (PNS), for which we reviewed the literature and contributed our observations under light and electron microscopy in this field. We consider the following aspects: (A) general characteristics of telocytes and the terminology used for these cells (e.g., endoneurial stromal cells) in PNS; (B) the presence, characteristics and arrangement of telocytes in the normal PNS, including (i) nerve epi-perineurium and endoneurium (e.g., telopodes extending into the endoneurial space); (ii) sensory nerve endings (e.g., Meissner and Pacinian corpuscles, and neuromuscular spindles); (iii) ganglia; and (iv) the intestinal autonomic nervous system; (C) the telocytes in the pathologic PNS, encompassing (i) hyperplastic neurogenic processes (neurogenic hyperplasia of the appendix and gallbladder), highly demonstrative of telocyte characteristics and relations, (ii) PNS tumours, such as neurofibroma, schwannoma, granular cell tumour and nerve sheath myxoma, and interstitial cell of Cajal-related gastrointestinal stromal tumour (GIST), (iii) tumour-invaded nerves and (iv) traumatic, metabolic, degenerative or genetic neuropathies, in which there are fewer studies on telocytes, e.g., neuroinflammation and nerves in undescended testicles (cryptorchidism), Klinefelter syndrome, crush injury, mucopolysaccharidosis II (Hunter’s syndrome) and Charcot–Marie–Tooth disease.